Item Details

abstract

Zinc oxide (ZnO) is a refractory semiconductor material whose band gap is both wide (3.39 eV) and direct. It is under consideration as a promising material for blue/uv semiconducting light emitting diodes (LEDs) and lasers. High purity macroscopic single crystals are available, along with epitaxial films grown by chemical vapor deposition, molecular beam epitaxy or pulsed-laser deposition. The exciton has a very fast spontaneous radiative lifetime, which makes it an interesting case for understanding what limits exciton oscillator strength. The 60 meV exciton binding energy is large relative to most other semiconductors, so exciton effects remain important at room temperature. In this thesis, results from time-dependent photoluminescence spectra of ZnO single crystals, films and powders from 16 to 296 K are reported. The main lifetime of the thermally shifted free exciton emission at room temperature is 440 ps. Temperature dependence shows this to be the radiative lifetime, and it decreases to 290 ps at 85 K. I measure the lifetime of the neutral donor bound exciton (D0X) in ZnO to be 50 ps. These are unusually fast spontaneous radiative lifetimes, and I have applied theories of exciton oscillator strength to understand the role of exciton coherence volume and look for possible ways to engineer very short lifetimes. I observed amplified spontaneous emission and so-called random media lasing in thin films and powder samples. The very short radiative lifetime for excitonic luminescence in this and other wide-gap semiconductors, and especially the “reverse quenching” temperature dependence of lifetime, can be explained in terms of the oscillator strength of the exciton, which scales with wavefunction size (coherence volume). The lifetime is then inversely proportional to the coherence volume of the exciton. This so-called giant oscillator strength has been described for CuCl and ZnO nanocrystals. Polariton transport was also studied in this dissertation. Energy transport at the D0X wavelength was found to occur at a velocity of 4.2 x 10^8 cm/s, and this was interpreted as polariton transport.

subject

photoluminescence

time-resolved

zinc oxide

ZnO

contributor

Wilkinson, John H (author)

jhwilkiv@netscape.net (authorEmail)

Kieth D. Bonin (committee member)

G. Eric Matthews (committee member)

Robert L. Swofford (committee member)

Eric Carlson (committee member)

Richard T. Williams (committee member)

creator

Wilkinson, John H

date

2008-09-28T10:54:09Z (accessioned)

2010-06-18T18:59:12Z (accessioned)

2007-08-11 (available)

2008-09-28T10:54:09Z (available)

2010-06-18T18:59:12Z (available)

2003-08-01 (issued)

degree

null (defenseDate)

Physics (discipline)

Wake Forest University (grantor)

PHD (level)

identifier

03_Wilkinson_PhD.pdf

http://hdl.handle.net/10339/14836 (uri)

migration

etd-09162005-083525 (oldETDId)

rights

Release the entire work immediately for access worldwide. (accessRights)

I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to Wake Forest University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. (license)

title

Picosecond Time-Resolved Photoluminescence of Zinc Oxide Single Crystals, Films and Nanoparticles